A long-term evolution (LTE) system that supports the 3rd Generation Partnership Project (3GPP) Release 8 standard and/or the 3GPP Release 9 standard has been developed by the 3GPP has a successor of a universal mobile telecommunications systems (UMTS) to enhance further performances of the UMTS so as to satisfy the increasing needs of users. Such LTE system would include a new radio interface and a new radio network architecture that provides high data rate, low latency, optimized data packet, improved system capacity, and improved system coverage. In a LTE system, a radio access network (RAN) known as an evolved universal terrestrial radio network (E-UTRAN) may include at least one evolved Node B (eNB) that would communicate with at least one multiple user equipment (UE). An eNB may also communicate with a core network through a mobility management entity (MME) to reach various network entities such as various gateways and servers within the non-access stratum (NAS) level of the core network.
A LTE-advanced (LTE-A) system, as its name implies, is a more advanced LTE system. A LTE-A communication system would provide, relative to its predecessors, faster switching between power states, better performance at the edge of the coverage of an eNB, and also advanced techniques such as carrier aggregation (CA), coordinated multipoint (CoMP) transmission/reception, UL multiple-input multiple-output (MIMO), and so forth. In order for a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must meet the standards developed for a LTE-A system such as the 3GPP Release 10 standard or later versions.
A small cell controlled by a low-power base station such as a micro cell base station, pico cell base station, femto cell base station, and so forth, could be deployed to satisfy increasing network traffic resulted from increasing demands of mobile electronic devices. A small cell could be deployed as hot spots for both indoor and outdoor scenarios. A low power base station could generally be defined as a base station having an operating power lower than that of a macro cell base station such as a macro NodeB, a macro eNB, or other variants. A UE may thus simultaneously communicate with a macro cell base station and a small cell base station by applying dual connectivity. In this instance, a UE may transmit or receive user data and control information via both the macro cell base station and the lower power base station. The dual connectivity may provide for the UE the benefit of an increased data throughput resulted from the simultaneous dual transmissions with a macro base station.
Any base station may serve two different roles as either a master eNB (MeNB) or a secondary eNB (SeNB), although typically a macro cell base station would be the MeNB and the small cell base station would be the SeNB. FIG. 1 illustrates a dual connectivity scenario applicable to a wireless communication system in which a UE is dual connected to two base stations. The wireless communication system could be a LTE or LTE-A communication system and would include at least but not limited to a MeNB 101, a SeNB 102, a UE 103. The MeNB 101 could be a macro cell base station and would provide a first coverage area 105, and the SeNB 102 could be a small cell base station and would provide a second coverage area 104 which is smaller than the first coverage area 105. The second coverage 104 could either overlap completely with the first coverage area 105 or could be partially outside of the first coverage area 105. For the wireless communication system of FIG. 1, the UE 103 would be able to communicate with both the MeNB 101 and the SeNB 102 at the same time. This means that, at some point in time, the UE 103 would be capable of performing an uplink transmission to both a cell of the MeNB 101 and a cell of the SeNB 102 as well as a downlink reception from both the cell of the MeNB 101 and the cell of the SeNB 102.
It should be noted that MeNB 101 and SeNB 102 may operate under different carrier frequencies. Hypothetically, assuming that a carrier aggregation scheme is used by the wireless communication system 100, which is for example a LTE-A communication system, MeNB 101 may operate a first component carrier (CC1) and SeNB may operate under a second component carrier (CC2), and the frequency spectrum of CC1 does not overlap with the frequency spectrum of CC2.
FIG. 2A illustrates a general RAN protocol architecture implemented by the exemplary wireless communication system of FIG. 1. The radio protocol architecture that a particular radio bearer uses depends on how the radio bearer is setup. A radio bearer can be a data radio bearer (DRB) for user plane data transmission and/or reception for a signaling radio bearer (SRB) for control plane data transmission and/or reception. A DRB for control plane includes DRB identity, EPS bearer identity, Packet Data Convergence Protocol (PDCP) configuration (e.g. header compression configuration), logical channel identity and logical channel configuration (e.g. priority and logical channel group). A SRB configuration includes SRB identity, Radio Link Control (RLC) configuration and logic channel configuration.
In dual connectivity, as shown in FIG. 2B, there could be at least three types of dual connectivity radio bearers: Master cell group (MCG) bearer 211, secondary cell group (SCG) 212 bearer and split bearer 213. The MCG bearer 211 utilizes radio protocols only located in MeNB (e.g. 101) to use MeNB resources only. SCG bearer 212 utilizes radio protocols only located in the SeNB (e.g. 102) to use SeNB resources. Split bearer 213 utilizes radio protocols located in both the MeNB (e.g. 101) and the SeNB (e.g. 102) to use both MeNB and SeNB resources. In general, a DRB can be a MCG bearer, a SCG bearer, or a split bearer. Also in general, a SRB can be a MCG bearer, a SCG bearer, or a split bearer.
A RB (i.e. DRB or SRB) is associated with one PDCP entity. The PDCP entity is associated with one or two (one for each direction) RLC entities depending on RB characteristic (i.e. uni-directional or bi-directional) and RLC mode (e.g. acknowledged mode or unacknowledged mode). For a split bearer, a PDCP entity is associated with two AM RLC entities. For encryption/decryption and integrity protection/check, a COUNT value associated to a PDCP service data unit (SDU) would be maintained by the UE side as well as base station(s) that provide services to the UE. The COUNT value is composed of a hyper frame number (HFN) and a PDCP sequence number. A state variable Next_PDCP_TX_SN indicates the PDCP sequence number of the next PDCP SDU to be transmitted by a PDCP entity. A state variable TX_HFN indicates HFN value for the generation of COUNT value used for encrypting or integrity protecting PDCP PDUs to be transmitted. A state variable Next_PDCP_RX_SN indicates the next expected PDCP sequence number by a receiving PDCP entity. A state variable RX_HFN indicates the HFN value for the generation of the COUNT value used for the received PDCP PDUs. Mechanisms involving transmissions and receptions of PDCP PDUs with the state variables as described above could be seen in 3GPP TS 36.323 which is incorporated by reference.
In dual connectivity, even though a DRB could be initiated as a MCG bearer, a SCG bearer, or a split bearer, the DRB could be reconfigured from one dual connectivity radio bearer into another dual connectivity radio bearer. In other words, after a MeNB sets up a DRB, the MeNB may reconfigure a DRB from a first dual connectivity radio bearer into a second dual connectivity radio bearer. Various possibilities are described below.
A first possibility involves a first SCG bearer that is changed into a second SCG bearer. That would occur in response to a DRB that has been configured as a first SCG bearer under a first SeNB but is reconfigured to be a second SCG bearer under a second SeNB because of a SeNB change.
A second possibility involves a SCG bearer that is changed into a MCG bearer. That would occur in response to a DRB that has been configured as a SCG bearer under a SeNB but is reconfigured to be a MCG bearer under a MeNB because of a circumstance such as the removal of the SeNB.
A third possibility involves a SCG bearer that is changed into a split bearer. That would occur in response to a DRB that has been configured as a SCG bearer under a SeNB but is reconfigured to be a split bearer under MeNB because of a circumstance such as offloading of traffic to the SeNB.
A fourth possibility involves a MCG bearer that is changed into a SCG bearer. That would occur in response to a DRB that has been configured as a MCG bearer under a MeNB but is reconfigured to be a SCG bearer under a SeNB because of a circumstance such as offloading traffic to the SeNB.
A fifth possibility involves a split bearer that is changed into a SCG bearer. That would occur in response to a DRB that has been configured as a split bearer but is reconfigured to be a SCG bearer because of a circumstance such as offloading traffic to the SeNB.
A sixth possibility involves a split bearer that is changed into a MCG bearer. That would occur in response to a DRB that has been configured as a split bearer but is reconfigured to be a MCG bearer because of a circumstance such as the removal of the SeNB.
A seventh possibility involves a first split bearer that is changed into a second split bearer. That would occur in response to a DRB that has been configured as a first split bearer but is reconfigured to be a MCG bearer because of a SeNB change.
An eighth possibility involves a MCG bearer that is changed into a split bearer. That would occur in response to a DRB that has been configured as a MCG bearer under MeNB but is reconfigured to be a split bearer because of a circumstance such as offloading traffic to a SeNB.
Mechanisms involving a DRB changing from one dual connectivity radio bearer into another dual connectivity radio bearer could be seen from references such as 3GPP TS 36.331 v12.1.0 and 3GPP R2-141857 which are incorporated by reference.
FIG. 3 illustrates a signaling diagram of a secondary cell group (SCG)/SeNB modification procedure. The SCG modification procedure would be initiated by a SeNB and would be used to perform configuration change of the SCG within the same SeNB. In step S301, a SeNB (e.g. SeNB 102) may transmit a SCG Modification Request message via the X2 interface application protocol (AP) in order to request a SCG modification by providing a new radio resource configuration of the SCG by a Radio Resource Control (RRC) container in the SCG Modification Request message. In step S302, in response to the SCG Modification Request message being received and accepted by a MeNB (e.g. MeNB 101), the MeNB may transmit to a UE (e.g. UE 103) a RRCConnectionReconfiguration message which includes the new radio resource configuration of SCG according to the SCG Modification Request message. In step S303, the UE may apply the new radio resource configuration of SCG and subsequently transmit a RRCConnectionReconfigurationComplete message back to the MeNB as a reply. Assuming that synchronization between the UE and the SeNB is not required to execute the new radio resource configuration of SCG the UE may perform UL transmissions after having applied the new configuration. If the new radio resource configuration of SCG requires synchronization between the UE and the SeNB, in step S305 the UE may initiate a Random Access (RA) procedure. In step S304. The MeNB may reply a SCG Modification Response to the SeNB by transmitting a RRCConnectionReconfigurationComplete message via the X2-AP.
Under the circumstance in which the UE is unable to comply with (part of) the radio resource configuration of SCG as included in the RRCConnectionReconfiguration message back in step S302, the UE may perform a reconfiguration failure procedure. The order of the UE transmitting the RRCConnectionReconfigurationComplete message and performing the RA procedure toward the SCG has not been defined. The success of the RA procedure towards the SCG is not required for a successful completion of the RRCConnectionReconfiguration procedure. The primary SCell (PSCell) in SCG could be changed with the SCG Modification procedure. The SeNB may decide whether the RA procedure is required according to, for example, whether an old special SCell or a new special SCell belongs to the same TAG The SeNB may trigger the release of SCG Scells(s) other than PSCell, and the MeNB cannot reject.
FIG. 4 illustrates a signaling diagram of a SCG addition/MeNB triggered modification procedure. The SCG addition procedure would typically be initiated by the MeNB to add the first cell of the SCG A MeNB may use a same or similar procedure to initiate an addition or a release of SCG cells and of SCG bearers. For all SCG modifications other than release of the entire SCG a SeNB would generate the signaling toward a UE. The MeNB may request to add particular SCells to the SeNB, and the SeNB may reject. By using the modification procedure, the MeNB may trigger the release of SCG SCells(s) other than pSCell, and in this case the SeNB cannot reject.
In step S401, the MeNB would transmit, via the X2-AP, a SCG addition/modification request message including a MCG configuration and (part of) UE capabilities for UE capability coordination to be used as a basis for the reconfiguration by the SeNB. In case of SCG addition and SCG SCell addition request, the MeNB can provide the latest measurement results (FFS for which SeNB cells). The SeNB may reject such request. In step S402, assuming that the SeNB accepts the MeNB request, the SeNB would initiate a SCG Modification procedure as specified in 10.1.2x.1 in R2-141857 and would provide a subsequent response which may be before or after the SCG Modification procedure. A SCG change procedure could be used to change a configured SCG of one SeNB to another SeNB in the UE and would be realized by a SCG Modification procedure. A RRCConnectionReconfiguration message transmitted during the SCG modification procedure would include information for the release of the source SCG.
In the aforementioned dual connectivity scenario, a UE would, at most of the time, connect to both a MeNB and a SeNB. However, as a UE is moving, the E-UTRAN that provides wireless service for the UE may change the MeNB and/or the SeNB for the UE. It should be noted that this scenario different from a typical handover scenario in which the UE is handover from a base station to another. For dual connectivity scenario, the UE would be simultaneously connected to both MeNB and SeNB and typically one of the MeNB and SeNB might change at a time.
However, during such period of changing the MeNB and/or SeNB for the UE, the data transmission and data reception would likely be either suspended or failed. The suspension or failure might impose potentially numerous difficulties for the network which attempts to implement lossless operations at all times. As an example, the encryption and decryption process would involve using a TX_HFN as shown in FIG. 2C. However, if a transmission has failed at TX_HFN=2 due to e.g. changing the SeNB to another eNB for the UE, it is uncertain what TX_HFN value should be used after the SeNB is changed to the other eNB.
Therefore, how to resume or recover data transmission and data reception after changing the MeNB and/or SeNB could be one of the design considerations as the current wireless technology moves forward.